Biodegradable polyesters

ABSTRACT

Biodegradable polyesters based on 
     A) 95-99.99 mol % of at least one polyester A containing as monomeric building blocks of an acid component comprising 
     a 11 ) 20-95 mol % of at least one aliphatic or cycloaliphatic dicarboxylic acid or its ester-forming derivative and 
     a 12 ) 5-80 mol % of at least one aromatic dicarboxylic acid or its ester-forming derivative and 
      at least one dihydroxy compound or at least one amino alcohol or their mixtures, and 
     B) 0.01-5 mol % of a mixture comprising mono-, bi-, tri-, tetra- and higher-nuclear isocyanurates or corresponding compounds containing two, three or four functional groups capable of reacting with the end groups of polyester A, or mixtures of the isocyanurates and the corresponding compounds, 
     as well as molding compositions comprising said polyesters, their manufacture and their use in the manufacture of moldings, filsm, fibers and coatings.

DESCRIPTION

The present invention relates to biodegradable polyesters obtainablefrom

A) 95-99.99 mol % of at least one polyester containing as monomericbuilding blocks

a₁) a mixture of

a₁₁) 20-95 mol % of at least one aliphatic or cycloaliphaticdicarboxylic acid or its ester-forming derivative and

a₁₂) 5-80 mol % of at least one aromatic dicarboxylic acid or itsester-forming derivative and

a₂) at least one dihydroxy compound or at least one amino alcohol ortheir mixtures

and

B) 0.01-5 mol % of a mixture comprising

b₁) 45-80% by weight of at least one mononuclear isocyanurate (b₁₁) orat least one compound (b₁₂) which contains two functional groups capableof reacting with the end groups of polyester A or mixtures of b₁₁ andb₁₂,

b₂) 13-25% by weight of at least one binuclear isocyanurate (b₂₁) or atleast one compound (b₂₂) which contains three functional groups capableof reacting with the end groups of polyester A or mixtures of b₂₁ andb₂₂,

b₃) 5-12% by weight of at least one trinuclear isocyanurate (b₃₁) or atleast one compound (b₃₂) which contains four functional groups capableof reaction with the end groups of polyester A or mixtures of b₃₁ andb₃₂ and

b₄) 2-18% by weight of at least one tetra- or higher-nuclearisocyanurate (b₄₁) or mixtures of different isocyanurates (b₄₁).

The present invention furthermore relates to molding compositions whichcomprise the biodegradable polyesters, to processes for preparing thebiodegradable polyesters and to their use for producing moldings, filmsor fibers. The invention furthermore relates to the moldings, films orfibers produced using the biodegradable polyesters.

Biodegradable polyesters which comprise repeating units derived from amixture of aromatic and aliphatic dicarboxylic acids or theirderivatives are disclosed, for example, in U.S. Pat. No. 5,446,079. Thisdescribes linear, random, semicrystalline polyesters having an intrinsicviscosity of about 0.5-1.8 dl/g (measured in phenol/tetrachloroethane,60/40 ratio by weight, at a concentration of 0.5 g/100 ml and at 25°C.), the dicarboxylic acid mixture comprising from 5 to 65 mol % ofaliphatic and from 35 to 95 mol % of aromatic dicarboxylic acids.

Chain-extended or branched polyesters are likewise known. Also known arepolyesters which are both branched and chain-extended. Diisocyanates arefrequently used to extend the chains, and polyfunctional acids and/oralcohols are often employed as branching agents. Isocyanates of higherfunctionality, polyisocyanates, are generally used as crosslinkers. Inorder to produce moldings with thick walls, U.S. Pat. No. 3,553,157recommends drum application of polyisocyanates before processing ontopolyethylene terephthalate (PET) so that the PET crosslinks duringprocessing. The molded articles produced in this way are, however, notfree of specks; the surface quality is thus unsatisfactory. U.S. Pat.No. 2,999,851 describes, for example, linear polyesters which arechain-extended with diisocyanates and can be crosslinked withpolyisocyanates. Polyesters modified in this way are, in particular,easily ground. However, they are not amenable to melt processing, norare they suitable for preparing polymer blends. WO 89/11497 disclosespolyesters from cycloaliphatic and aromatic monomers, which are branchedin a first step and reacted with polyisocyanates in another step. Thesepolyesters can be processed to films. A disadvantage is, however, thatthese polyesters are not thermoplastic. The films must therefore beproduced by rolling. Production of satisfactory films by blow molding isnot possible with these polyesters. Adhesives made fromaliphatic/aromatic polyesters chain-extended with diisocyanates andcrosslinked with polyisocyanates are disclosed in U.S. Pat. No.3,804,810.

EP-A1-572 682 discloses that biodegradable films can be obtained fromaliphatic polyesters which have, for example, been branched withpyromellitic dianhydride and chain-extended with polyisocyanates.Aliphatic/aromatic polyesters chain-extended with diisocyanates andcapable of biodegradation are disclosed in DE-A1-44 40 858.

The biodegradable polyesters disclosed to date do not yet meet allrequirements, in particular for producing films. In addition, thealiphatic polyesters show, although chain-extended and branched, atendency to stick. Furthermore, they have the disadvantage that theirmelting points are too low and thus they have inadequate heatresistance. It is true that succinic acid can be employed to raise themelting points. However, this dicarboxylic acid is too costly for massproduction. The linear, aliphatic/aromatic polyesters chain-extendedwith diisocyanates can be processed to films better than thecorresponding ones without chain extension. On the other hand, they formgel particles which interfere with processing, especially when the cycletimes are long.

It is an object of the present invention to develop biodegradablearomatic/aliphatic polyesters which can be processed to films, which arenot sticky and which have good surface quality. Furthermore,aromatic/aliphatic polyesters shall be provided whose mechanicalproperties resemble polyesters having a high proportion of aromaticunits, but whose degradability is similar to that of polyesters havinghigh proportions of aliphatic units. It was particularly intended thatthe novel polyesters make it possible for the cycle times to be long orvery long during processing, also in the production of moldings, forexample by injection molding.

It was furthermore intended to find a material which can be processed tobiodegradable filaments. It was furthermore intended to make availablebiodegradable polyesters which can be processed with other materials,especially those which are themselves biodegradable, to moldingcompositions.

We have found that this object is acheived by the polyesters mentionedat the outset.

The term "biodegradable" as used within the scope of the presentapplication refers to the fact that the polyesters decompose underenvironmental influences in an appropriate and demonstrable timespan.This degradation usually takes place by hydrolysis and/or oxidation, butmainly by the action of microorganisms such as bacteria, yeasts, fungiand algae. However, enzymatic degradation is also possible, asdescribed, for example, by Y. Tokiwa and T. Suzuki in "Nature" 270(1977) 76-78. It is moreover possible within the scope of the presentinvention to alter the rate of biodegradation, ie. the time taken forthe polyesters according to the invention to be essentially completelydegraded, by the appropriate choice of the ratio between repeating unitsderived from aliphatic carboxylic acids or their ester-formingderivatives and those derived from aromatic carboxylic acids or theirester-forming derivatives. The rule of thumb applying to this is thatthe rate of biodegradation of the polyesters increases with the contentof repeating units derived from aliphatic carboxylic acids or theirester-forming derivatives. Furthermore, the rate of biodegradation ofthe polyesters increases with the content of sections with analternating sequence of repeating units derived from aliphatic andaromatic carboxylic acids or their ester-forming derivatives.

Polyester A

According to the invention, the polyesters A comprise a mixture a₁ ofthe dicarboxylic acids a₁₁ and a₁₂ and dihydroxy compounds a₂.

The aliphatic dicarboxylic acids a₁₁ suitable according to the inventionfor preparing polyesters A generally have 2 to 10 carbon atoms,preferably 4 to 6 carbon atoms. They may be either linear or branched.The cycloaliphatic dicarboxylic acids a₁₁ which can be used for thepurpose of the present invention are, as a rule, those having 7 to 10carbon atoms and, in particular, those having 8 carbon atoms. It is alsopossible in principle, however, to employ dicarboxylic acids a₁₁ havinga larger number of carbon atoms, for example up to 30 carbon atoms.

Examples which may be mentioned are: malonic acid, succinic acid,glutaric acid, adipic acid, pimelic acid, azelaic acid, sebacic acid,fumaric acid, 2,2-dimethylglutaric acid, suberic acid,1,3-cyclopentanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid,1,3-cyclohexanedicarboxylic acid, diglycolic acid, itaconic acid, maleicacid and 2,5-norbornanedicarboxylic acid, of which adipic acid orsebacic acid is preferred.

Ester-forming derivatives of the abovementioned aliphatic orcycloaliphatic dicarboxylic acids all which should be particularlymentioned are the di-C₁ -C₆ -alkyl esters such as dimethyl, diethyl,di-n-propyl, diisopropyl, di-n-butyl, diisobutyl, di-t-butyl,di-n-pentyl, diisopentyl or di-n-hexyl esters. It is likewise possibleto employ anhydrides of the dicarboxylic acids a₁₁.

It is moreover possible to employ the dicarboxylic acids or theirester-forming derivatives a₁₁ singly or as mixture of two or morethereof.

Preferably employed are adipic acid or its ester-forming derivativesalone or sebacic acid or its ester-forming derivatives alone or mixturesof adipic acid and sebacic acid or their ester-forming derivatives,especially adipic acid or its ester-forming derivatives alone.

The content of aliphatic or cycloaliphatic dicarboxylic acid or itsester-forming derivatives a₁₁ is, according to the invention, from 20 to95, preferably from 30 to 70, particularly preferably from 40 to 65, inparticular from 50 to 60, mol %, in each case based on the total amountof components a₁₁ and a₁₂.

Aromatic dicarboxylic acids a₁₂ which should generally be mentioned arethose having 8 to 12 carbon atoms and, preferably, those having 8 carbonatoms. Examples which may be mentioned are terephthalic acid,isophthalic acid, 2,6-naphthalic acid and 1,5-naphthalic acid, andester-forming derivatives thereof. Particular mention should be made inthis connection of the di-C₁ -C₆ -alkyl esters, eg. dimethyl, diethyl,di-n-propyl, diisopropyl, di-n-butyl, diisobutyl, di-t-butyl,di-n-pentyl, diisopentyl or di-n-hexyl ester. The anhydrides of thedicarboxylic acids a₁₂ are likewise suitable ester-forming derivatives.Here preference is given to terephthalic acid or its ester-formingderivatives, especially to the dimethyl ester, or mixtures thereof.

However, it is also possible in principle to employ aromaticdicarboxylic acids a₁₂ having a larger number of carbon atoms, forexample up to 20 carbon atoms.

The aromatic dicarboxylic acids or their ester-forming derivatives a₁₂can be employed singly or as mixture of two or more thereof.

The content of aromatic dicarboxylic acids or their ester-formingderivatives a₁₂ is, according to the invention, from 5 to 80, preferablyfrom 30 to 70, particularly preferably from 35 to 60, in particular from40 to 50, mol %, in each case based on the total amount of componentsa₁₁ and a₁₂.

Employed as component a₂ according to the invention is at least onedihydroxy compound or at least one amino alcohol or mixtures thereof. Itis possible in principle to use all diols or amino alcohols able to formesters with the dicarboxylic acids a₁₁ or a_(l2).

However, in general, branched or linear alkanediols having 2 to 12carbon atoms, preferably 4 to 6 carbon atoms, or cycloalkanediols having5 to 10 carbon atoms, a₂₂ polyetherdiols, ie. dihydroxy compoundscontaining ether groups, or amino alcohols having 2 to 12 carbon atoms,preferably 2 to 4 carbon atoms, and cyclic amino alcohols having 5 to 10carbon atoms, are employed as component a₂.

Examples of suitable alkanediols are ethylene glycol, 1,2-propanediol,1,3-propanediol, 1,2-butanediol, 1,4-butanediol, 1,5-pentanediol,2,4-dimethyl-2-ethyl-1,3-hexanediol, 2,2-dimethyl-1,3-propanediol,2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-isobutyl-1,3-propanediol,2,2,4-trimethyl-1,6-hexanediol, in particular ethylene glycol,1,3-propanediol, 1,4-butanediol or 2,2-dimethyl-1,3-propanediol(neopentyl glycol); cyclopentanediol, 1,4-cyclohexanediol,1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol,1,4-cyclohexanedimethanol or 2,2,4,4-tetramethyl-1,3-cyclobutanediol. Itis also possible to use mixtures of different alkanediols.

Examples of polyetherdiols are diethylene glycol, triethylene glycol,polyethylene glycol, polypropylene glycol, polytetrahydrofuran,especially diethylene glycol, triethylene glycol, polyethylene glycol,or mixtures thereof, or compounds having a different number of etherunits, for example polyethylene glycol containing propylene units andobtainable, for example, by polymerizing, by conventional methods, firstethylene oxide and then propylene oxide. The molecular weight (Mn) ofthe polyethylene glycols which can be employed is, as a rule, from about250 to about 8000, preferably from about 600 to about 3000, g/mol.Mixtures of different polyetherdiols can likewise be used.4-Aminomethylcyclohexanemethanol, 2-aminoethanol, 3-aminopropanol,4-aminobutanol, 5-aminopentanol, 6-aminohexanol; aminocyclopentanol andaminocyclohexanol, or mixtures thereof, are examples of amino alcoholswhich can be employed as component a₂. Mixtures of different aminoalcohols can likewise be employed.

The molar ratio of a₁ to a₂ is generally chosen in the range from 0.4:1to 2.5:1, preferably in the range from 0.5:1 to 1.5:1, furthermorepreferably in the range from 0.5:1 to 1.2:1 and, in particular, in therange from 0.8:1 to 1.1:1.

Preferred polyesters A contain the monomeric building blocks a₁ and a₂.The molar ratio of a₁ and a₂ in the isolated polyester A is, afterremoval of the required amount of excess component a₂, generallyapproximately 0.8:1 to about 1:1, preferably within the range from 0.9to 1:1, particularly preferably within the range from 0.95 to 1:1. Thepolyesters A preferably contain hydroxyl and/or amino end groups, ie.,a₁ :a₂ is somewhat less than 1.

In general, polyester A is essentially linear and its weight averagemolecular weight Mw is generally within the range from 5000 to 120,000g/mol, preferably within the range from 20,000 to 80,000 g/mol,determined by gel permeation chromatography using polystyrene standardswith narrow distributions in tetrahydrofuran as solvent. Thepolydispersity (PDI=M_(w) /M_(n)) of polyesters A is generally withinthe range from 2 to 4, preferably within the range from 2.5 to 3.5.

Component B

Mixture B of the invention comprises a component b₁ in proportions from45 to 80, preferably from 50 to 75, for example from 55 to 70, % byweight, based on the total weight of mixture B. Mixture B of theinvention further comprises from 13 to 25, preferably from 15 to 23,especially from 16 to 20, % by weight, based on the total weight ofmixture B, of a component b₂. Component b₃ is present in mixture B in aproportion from 5 to 12, preferably from 5 to 11, % by weight, based onthe total weight of mixture B. For example, B may comprise from 6 to 10%by weight, based on the total weight of mixture B, of b₃. As well ascomponents b₁ and b₃, mixture B comprises from 2 to 18, preferably from5 to 16, % by weight, based on the total weight of mixture B, ofcomponent b₄. In a preferred embodiment, B comprises from 8 to 15% byweight, based on the total weight of mixture B, of component b₄.

Components b₁ to b₄ are each composed of at least one isocyanurate or atleast one compound having groups which are reactive with the end groupsof polyester A, or mixtures thereof. Components b₁ to b₄ differ in thatthe isocyanurates b₁₁ to b₄₁ have a differing number of nuclei whichindicates the number of cyanurate rings in the molecule. According tothe invention, component b₁ may comprise mononuclear isocyanurates, b₂may comprise binuclear isocyanurates, b₃ may comprise trinuclearisocyanurates and b₄ may comprise tetra- or higher-nuclearisocyanurates. Said isocyanurates b₁₁ to b₄₁ can have the same basicstructure in all components b₁ to b₄. However, the isocyanurates incomponents b₁ to b₄ can also have different basic structuresindependently of one another. B₁ to b₄ further differ in the number offunctional groups in the reactive compounds b₁₂ to b₄₂. The basicmolecule of this compound can be identical in the four components b₁ tob₄ or independently different from one another.

In a preferred embodiment, the isocyanurates b₁₁ to b₄₁ are used alonein components b₁ to b₄. The preferred isocyanurates include thealiphatic isocyanurates, such as isocyanurates derived from alkylenediisocyanates or cycloalkylene diisocyanates having from 2 to 20 carbonatoms, preferably from 3 to 12 carbon atoms, eg. isophoronediisocyanate. The alkylene diisocyanates can be not only linear but alsobranched. Particular preference is given to diisocyanurates which arebased on n-hexamethylene diisocyanate.

As component B, however, it is also possible to use mixtures comprisingdifunctional, trifunctional and tetrafunctional epoxides b₁₂ to b₃₂.Bisphenol A diglycidyl ether is an example of a suitable difunctionalepoxide. Suitable trifunctional epoxides include1,3,5-trisoxiranylmethyl[1.3.5]triazinane-2,4,6-trioxane,2,4,6-trisoxiranyl[1.3.5]trioxane, trisoxiranylmethylbenzene-1,3,5-tricarboxylate or 1,1,1-tris(hydroxymethyl)ethanetris(glycidyl ether), 1,1,1-tris(hydroxymethyl)propane tris(glycidylether). 4,4'-Methylenebis[N,N-bis(2,3-epoxypropoyl)aniline] orpentaerythritol tetraglycidyl ether can be used as tetra-functionalepoxide.

Mixture B, which comprises isocyanurates only in a preferred embodiment,generally has a viscosity of from 100 to 20,000 mPas. Preferred mixturesB have viscosities in the range from 500 to 15,000 mpas. Theparticularly preferred mixtures include mixtures having viscosities inthe range from 2000 to 6000 mpas. However, also suitable are mixtureshaving lower viscosities, for example in the range from 500 to 1000. Itis furthermore possible to use, for example, mixtures whose viscositiesare in the range from 9000 to 13,000. The stated viscosities relate ineach case to measurements by the DIN 53 019 method at 23° C.

Besides the isocyanate groups and the functional groups of compounds b₁₂to b₄₂, mixtures B may further comprise small amounts of, in general,not more than 5% by weight, based on the total of components b₁ to b₄,of further compounds having allophanate or uretdione groups.

In general, the mixtures B are prepared by catalytic oligomerization ofaliphatic diisocyanates. Suitable catalysts include, for example,hydroxides or organic salts of weak acids with tetraalkylammoniumgroups, hydroxides or organic salts of weak acids withhydroxyalkylammonium groups, alkali metal salts of tin, zinc or leadsalts of alkylcarboxylic acids. It is further possible to use, ascatalysts, alkali metal alkoxides and phenoxides, metal salts ofcarboxylic acids, for example cobalt naphthenate, sodium benzoate,sodium acetate and potassium formate, tertiary amines, for exampletriethylamine, N,N-dimethylbenzylamine, triethylenediamine,tris-2,4,6-(dimethylaminomethyl)phenol,tris-1,3,5-(dimethylaminopropyl)-S-hexahydrotriazine, tertiaryphosphines or tertiary ammonium compounds.

This is accomplished by allowing the aliphatic and/or cycloaliphaticdiisocyanates to react in the presence of a catalyst, optionally withthe use of solvents and/or assistants, until the desired conversion hasbeen reached. In general, the reaction is carried out at temperatureswithin the range from 60 to 100° C. Thereafter the reaction isdiscontinued by deactivation of the catalyst and excess monomericdiisocyanate is distilled off. The catalyst can be deactivated byaddition of an aliphatic dicarboxylic acid, eg., oxalic acid orethylhexanoic acid, or by heating the reaction mixture to temperaturesabove 100° C. Depending on the catalyst type used and on the reactiontemperature, polyisocyanates are obtained that have differentproportions of isocyanurate and uretdione groups.

Such processes are known per se, for example from DE-A 43 20 821 and 4405 055.

The products thus prepared are usually clear products which, however,depending on the catalyst type, the diisocyanate grade, the reactiontemperature and the reaction procedure have a more or less pronouncedyellow color.

The biodegradable polyesters of the invention are branched and, if atall, crosslinked only to a minor extent. The polyesters of the inventiongenerally have a weight average molecular weight Mw within the rangefrom 60,000 to 250,000 g/mol, for example within the range from 70,000to 200,000 g/mol. Depending on the desired property profile, themolecular weights (weight average Mw) can be for example within thelower range, for example within the range from 60,000 to 140,000,preferably within the range from 80,000 to 110,000, g/mol. However, theycan also be in the higher molecular range, for example, in a preferredembodiment, within the range from 180,000 to 200,000 g/mol (weightaverage Mw). In a further preferred embodiment, the polyesters of theinvention have molecular weights (weight average Mw) within the rangefrom 150,000 to 160,000 g/mol. The molecular weights are determined bygel permeation chromatography (polystyrene standards with narrowdistributions and tetrahydrofuran as solvent). Their polydispersityindex (PDI=Mw/Mn) is in the range from 2 to 9, preferably 3 to 7.

The biodegradable polyesters according to the invention can be usedwithout other additives or be employed as molding compositions mixedwith appropriate additives.

It is possible to add from 0 to 80% by weight of additives, based on thebiodegradable polyesters according to the invention. Examples ofsuitable fillers are carbon black, starch, lignin powder, cellulosefibers, natural fibers such as sisal and hemp, iron oxides, clayminerals, talc, ores, calcium carbonate, calcium sulfate, barium sulfateand titanium dioxide. The additives may also in some cases containstabilizers such as tocopherol (vitamin E), organic phosphoruscompounds, mono-, di- and polyphenols, hydroquinones, diarylamines,thioethers, UV stabilizers, nucleating agents such as talc, andlubricants and mold release agents based on hydrocarbons, fattyalcohols, higher carboxylic acids, metal salts of higher carboxylicacids, such as calcium and zinc stearates, and montan waxes. Stabilizersof these types are described in detail in the Kunststoff-Handbuch,Volume 3/1, Carl Hanser Verlag, Munich, 1992, pages 24 to 28.

The biodegradable polyesters according to the invention may additionallybe colored as desired by adding organic or inorganic dyes. The dyes mayalso be regarded as additives in the widest sense.

Depending on the required application, the biodegradable polyestersaccording to the invention may be mixed with thermoplastic polymers orother polymers which are themselves biodegradable.

Preferred thermoplastic polymers with which the biodegradable polyestersaccording to the invention can be processed to give molding compositionsare those compatible with the biodegradable polyesters according to theinvention. These include in particular polycarbonates, polyacrylates orpolymethacrylates, preferably poly(methyl methacrylate) or polyvinylacetate.

The biodegradable polyesters according to the invention may beprocessed, for example, with cellulose compounds such as celluloseesters, in particular cellulose alkyl esters, such as cellulose acetate,cellulose propionate, cellulose butyrate or mixed cellulose esters, eg.cellulose acetate butyrate or cellulose propionate butyrate, to givemolding compositions.

It is furthermore possible to employ the biodegradable polyestersaccording to the invention mixed with other polyesters which arethemselves biodegradable, for example the linear aliphatic/aromaticcopolyesters disclosed in U.S. Pat. No. 5,446,079 or thealiphatic/aromatic polyesters described in DE-A1-44 40 858.

It is furthermore possible to prepare molding compositions which containthe biodegradable polyesters according to the invention plusbiodegradable aliphatic polyesters. The biodegradable aliphaticpolyesters which can be used as blend component include both linearaliphatic polyesters without chain extensions and those having chainextensions and/or branches. Particularly preferred aliphatic polyestersare those such as polyhydroxybutyrate, copolymers based onpolyhydroxybutyrate and polyhydroxyvalerate, poly(hexamethyleneglutarate), poly(hexamethylene adipate), poly(butylene adipate),poly(butylene succinate), poly(ethylene adipate), poly(ethyleneglutarate), poly(diethylene adipate), poly(diethylene succinate) orpoly(hexamethylene succinate). The preferred blend components alsoinclude the abovementioned branched aliphatic polyesters with chainextensions disclosed in EP-A1-572 682.

The biodegradable polyesters according to the invention may furthermorebe mixed with starch, preferably modified starch, in particularthermoplastic starch or starch compounds such as starch ethers or starchesters.

It is, of course, possible for the molding compositions based on thebiodegradable polyesters according to the invention and thethermoplastic or other biodegradable polymers to contain additives suchas the abovementioned fillers, dyes, stabilizers or processing aids inthe stated amounts. These molding compositions may furthermore, ifnecessary, contain additives typical of polymer blends, for examplecompatibilizers.

The biodegradable polyesters can be obtained according to the inventionby, in a first step, preparing the polyesters A and, in a second step,reacting from 95 to 99.99, preferably from 97 to 99.95, particularlypreferably from 99.0 to 99.90, mol % of the polyesters A with from 0.01to 5, preferably from 0.02 to 3, particularly preferably from 0.03 to1.5, mol %, in particular from 0.05 to 1.0 mol % of the mixture B, wherethe stated mol % is based on the total of components A and B.

Processes for preparing polyesters A are known in principle (Sorensenand Campbell, "Preparative Methods of Polymer Chemistry", IntersciencePublishers, Inc., New York, 1961, pages 111 to 127; Encycl. of Polym.Science and Eng., Vol. 12, 2nd Ed., John Wiley & Sons, 1988, pages 1 to75, Kunststoff-Handbuch, Volume 3/1, Carl Hanser Verlag, Munich, 1992,pages 15 to 23 (preparation of polyesters); WO 92/13019; EP-A 568 593;

EP-A 565, 235; EP-A 28,687), so that detailed statements on this areunnecessary.

Thus, for example, it is possible to react dimethyl esters of componenta₁ with component a₂ ("transesterification") at from 160 to 230° C. inthe melt under atmospheric pressure, advantageously under an inert gasatmosphere.

It is advantageous to use for preparing the polyester A a molar excessof component a₂ relative to component a₁, for example up to 2.5-fold,preferably up to 1.7-fold.

When dicarboxylic acids or their anhydrides are used as component a₁,esterification thereof with component a₂ can take place before, at thesame time as or after the transesterification.

In a preferred embodiment, the process for preparing modifiedpolyalkylene terephthalates described in DE-A 23 36 026 is used.

After reaction of components a₁ and a₂ and if desired a₃, as a rule thepolycondensation is carried out under reduced pressure or in a stream ofinert gas, for example of nitrogen, with further heating at from 180 to260° C. until the molecular weight is that required.

In order to avoid unwanted degradation and/or side reactions, ifrequired stabilizers can also be added in this stage. Examples of suchstabilizers are the phosphorus compounds described in EP-A 13 461, U.S.Pat. No. 4,328,049 or in B. Fortunato et al., Polymer Vol. 35, No. 18,pages 4006 to 4010, 1994, Butterworth-Heinemann Ltd. These may also insome cases act as deactivators of the catalysts described above.Examples which may be mentioned are: organophosphites, phosphonous acidand phosphorous acid. Examples of compounds which act only asstabilizers and which may be mentioned are: trialkyl phosphites,triphenyl phosphite, trialkyl phosphates, triphenyl phosphate andtocopherol (vitamin E; for example obtainable as Uvinul® 2003AO (BASF)).

Polyester A is normally prepared with the addition of suitable, knowncatalysts such as metal compounds based on elements such as Ti, Ge, Zn,Fe, Mn, Co, Zr, V, Ir, La, Ce, Li and Ca, preferably organometalliccompounds based on these metals, such as salts of organic acids,alkoxides, acetylacetonates and the like, particularly preferably basedon zinc, tin and titanium.

When the biodegradable polyesters according to the invention are usedfor example in the packaging sector, eg. for foodstuffs, it is usuallydesirable to choose the content of catalyst employed to be as low aspossible, and not to employ toxic compounds. In contrast to other heavymetals such as lead, tin, antimony, cadmium, chromium etc., as a ruletitanium and zinc compounds are non-toxic ("Sax Toxic Substance DataBook", Shizuo Fujiyama, Maruzen, K. K., 360 S. (quoted in EP-A 565 235),see also Rompp Chemie Lexikon, Vol. 6, Thieme Verlag, Stuttgart, NewYork, 9th Edition, 1992, pages 4626 to 4633 and 5136 to 5143). Exampleswhich may be mentioned are: dibutoxydiacetoacetoxy-titanium, tetrabutylorthotitanate and zinc(II) acetate.

The ratio by weight of catalyst to polyester A is normally in the rangefrom 0.01:100 to 3:100, preferably from 0.05:100 to 2:100, it also beingpossible to employ smaller amounts of highly active titanium compounds,such as 0.0001:100.

The catalyst can be employed right at the start of the reaction,immediately before the removal of the excess dihydroxy compound or else,if required, divided in several portions, during the preparation of thebiodegradable polyesters A. It is also possible, if required, to employdifferent catalysts or else mixtures thereof.

Polyesters A are preferably reacted with mixture B in the melt, it beingnecessary to take care that, if possible, no side reactions possiblyleading to crosslinking or gel formation take place. This isaccomplished according to the invention by means of the selectedquantitative ranges for mixture B. Similarly, very thorough mixingserves to minimize secondary reactions. In a particular embodiment, thereaction is normally carried out at from 140 to 260, preferably from 180to 250° C., with the mixture advantageously being added in severalportions or continuously.

If required, polyester A can also be reacted with mixture B in thepresence of conventional inert solvents such as toluene, methyl ethylketone or dimethylformamide (DMF) or mixtures thereof, in which case thereaction temperature is, as a rule, chosen in the range from 80 to 200,preferably from 90 to 150° C.

The reaction with mixture B can be carried out batchwise orcontinuously, for example in stirred vessels, static mixers or reactionextruders.

It is also possible to employ conventional catalysts known from theprior art (for example those described in EP-A 534,295) for reactingpolyester A with mixture B.

Examples which may be mentioned are: tertiary amines such astriethylamine, dimethylcyclohexylamine, N-methylmorpholine,N,N'-dimethylpiperazine, diazabicyclo[2.2.2]octane and the like, and, inparticular, organometallic compounds such as titanium compounds, ironcompounds, tin compounds, eg. dibutoxy-diacetoacetoxytitanium,tetrabutyl orthotitanate, tin diacetate, dioctoate, dilaurate or thedialkyltin salts of aliphatic carboxylic acids such as dibutyltindiacetate, dibutyltin dilaurate or the like, it again being necessary totake care that, where possible, no toxic compounds are employed.

The biodegradable polyesters according to the invention are veryparticularly preferably prepared by initially polymerizing the polyesterA for example in a vessel or a continuous polycondensation system asdescribed above. The polyester A obtained this way is, without furtherpurification, discharged into a mixing apparatus which permits verythorough mixing of the viscous polyester A and of the low viscositymixture B. Examples of suitable mixing apparatus are extruders such asreaction extruders which have a metering apparatus, preferably in thefirst extruder half, in particular the first extruder third, or staticmixers. However, it may also be advantageous, before polyester A istransferred into the mixing apparatus such as the static mixer, topurify polyester A in an intermediate step.

Mixture B can be metered in upstream of the mixing element or intocomponent A in the mixing element. In a particularly preferredembodiment, component B is, before introduction of component A into theextruder or the static mixer, continuously fed into the componentsdischarged from the vessel or polymerization system. The twopart-streams are homogenized in the static mixer and react at the sametime. The melt temperatures in the static mixer are particularlypreferably in the range from 190 to 240° C.

The polymer melt is then discharged, for example by a caster or a holedie, and either granulated under water or cooled in a cooling bath andthen granulated. However, a belt granulator can also be employed for thegranulation step, for example.

The biodegradable polyesters according to the invention, and the moldingcompositions containing them, can be applied to coating substrates byrolling, painting, spraying or pouring. Preferred coating substrates arethose which can be composted or rot, such as moldings of paper,cellulose or starch.

The biodegradable polyesters according to the invention, and the moldingcompositions, can additionally be used to produce moldings which can becomposted. Examples of moldings which may be mentioned are: disposablearticles such as dishes, cutlery, refuse sacks, films for agriculturefor harvest advancement, or for protection from moisture as underfilms,fango packs or tablecloths, as packaging films and vessels for growingplants or as tire material, disposable coathangers like those requiredin dry cleaners, as disposable gloves or medical syringes.

Furthermore, the biodegradable polyesters according to the invention,and the molding compositions containing them, can be spun to filamentsin a manner known per se. The filaments can, if required, be drawn,draw-twisted, draw-wound, draw-warped, draw-sized and draw-texturized,by conventional methods. The drawing to flat yarn can moreover becarried out in one and the same operation (fully drawn yarn or fullyoriented yarn) or in a separate operation. The draw-warping, draw-sizingand draw-texturizing is generally carried out in an operation separatefrom the spinning. The filaments can be further processed to fibers in amanner known per se. It is then possible to obtain fabrics from thefibers by weaving or knitting. The filaments can be used, for example,for closing disposable articles or for producing dental floss, or towscan be used for cigarette filters; nonwovens are used in the hygienesector or in the household.

A particular area of application of the biodegradable polyestersaccording to the invention, and of the molding compositions containingthese polyesters, relates to use as compostable film or a compostablecoating as outer layer of diapers. The outer layer of the diaperseffectively prevents the passing through of liquids absorbed in theinterior of the diapers by the fluff and superabsorbents, preferably bybiodegradable superabsorbents, for example based on crosslinkedpolyacrylic acid or crosslinked polyacrylamide. A nonwoven cellulosematerial can be used as inner layer of the diapers. The outer layer ofthe described diapers is biodegradable and thus compostable. Itdecomposes on composting so that the complete diaper rots, whereasdiapers provided with an outer layer of, for example, polyethylenecannot be composted without prior reduction in size or elaborate removalof the polyethylene sheet.

Another preferred use of the biodegradable polyesters according to theinvention, and of the molding compositions containing these polyesters,relates to production of adhesives in a manner known per se (see, forexample, Encycl. of Polym. Sc. and Eng. Vol. 1 "Adhesive Compositions",pages 547 to 577). The polymers and molding compositions according tothe invention can also be processed with suitable tackifyingthermoplastic resins, preferably natural resins, by methods described inEP-A 21042. The polymers and molding compositions according to theinvention can also be further processed to solvent-free adhesive systemssuch as hot-melt films in a similar way to that disclosed in DE-A4,234,305. It is moreover possible to produce, for example, plasticfilms which can be inscribed and which adhere to paper but can bedetached again to allow marking of desired places in the text.

Another preferred area of application relates to the production ofcompletely degradable blends with starch mixtures (preferably withthermoplastic starch as described in WO 90/05161) similar to the processdescribed in DE-A 42 37 535. The polymers and thermoplastic moldingcompositions according to the invention can, according to observationsto date, because of their hydrophobic nature, their mechanicalproperties, their complete biodegradability, their good compatibilitywith thermoplastic starch and, last but not least, because of theirfavorable raw materials basis, be advantageously employed as syntheticblend component.

Further areas of application relate, for example, to the use of thebiodegradable polyesters according to the invention, and of the moldingcompositions containing these polyesters, in agricultural mulch,packaging material for seeds, nutrients or flowers, substrate inadhesive films, underpants for babies, handbags, bed films, bottles,boxes, dust bags, labels, cushion covers, protective clothing, hygienearticles, handkerchiefs, toys and wipes.

The biodegradable polyesters according to the invention aredistinguished by containing deliberately incorporated branching pointsbut being uncrosslinked and thus remaining thermoplastic and readilydegradable under conditions like those prevailing in compost.

The advantages of the biodegradable polyesters according to theinvention compared with other known biodegradable polyesters based onaliphatic and aromatic dicarboxylic acids lie in the good surfacequality of the moldings or films produced from the polyesters accordingto the invention or the molding compositions containing them. Thesurfaces are very substantially free of bits or specks. In addition,they have no defects derived from gel formation. The biodegradablepolyesters according to the invention are furthermore suitable forproducing films or moldings in conventional molding machines operatingwith long cycle times. It is possible to produce very thin films at acomparatively low level of aromatic building blocks in the polyester.Furthermore, the biodegradable polyesters of the invention carry theirfavorable film properties into the blends produced therefrom.

Furthermore, the biodegradable polyesters according to the inventionsurprisingly display pseudoplasticity, ie. viscosity depending on therate of shear, which is greater than that of polyesters of correspondingstructure and chain length which have branching points and are derivedfrom trifunctional branching agents such as trifunctional alcohols orpure trifunctional isocyanates. The biodegradable polyesters accordingto the invention therefore have greater mechanical melt stability evenwith a lower degree of branching than in the case of branching withtrifunctional branching agents.

Compared with linear biodegradable polyesters, the breaking extension ofthe biodegradable polyesters according to the invention is betterbecause it is balanced in the longitudinal and transverse directions.The polyesters of the invention moreover have similar breaking strengthsand yield stresses to linear biodegradable polyesters having acomparable level of groups derived from isocyanates while, at the sametime, having lower breaking extensions. They are therefore particularlysuitable for the production of blown films.

EXAMPLES

The melt volume index was measured in accordance with ISO 1133 under aload of 2.16 kg and at a temperature of 190° C.

The hydroxyl number (OH number) and the acid number (AN) were determinedaccording to the following methods:

(a) Determination of the Apparent Hydroxyl Number

To about 1 to 2 g of accurately weighed-out test substance were added 10ml of toluene and 9.8 ml of acetylating reagent (see below) and heatedat 95° C. for 1 hour with stirring. The reafter 5 ml of distilled waterwere added. After cooling down to room temperature, 50 ml oftetrahydrofuran (THF) were added and potentiographically titrated withethanolic KOH standard solution to the turning point.

The run was repeated without test substance (blank sample).

The apparent OH number was then determined on the basis of the followingformula:

    apparent OH number c.t.56.1.(V2-V1)/m (in mg of KOH/g)

where

c=concentration of ethanolic KOH standard solution in mol/l,

t=titer of ethanolic KOH standard solution

m=weight in mg of test substance

V1=consumption of standard solution with test substance in ml

V2=consumption of standard solution without test substance in ml.

Reagents used:

ethanolic KOH standard solution, c=0.5 mol/l, titer 0.9933 (Merck, Art.No. 1.09114)

acetic anhydride p.A. (Merck, Art. No. 42)

pyridine p.A. (Riedel de Haen, Art. No. 33638)

acetic acid p.A. (Merck, Art. No. 1.00063)

acetylating reagent: 810 ml of pyridine, 100 ml of acetic anhydride and9 ml of acetic acid

water, deionized

THF and toluene

(b) Determination of the Acid Number (AN)

About 1 to 1.5 g of test substance were weighed out accurately andadmixed with 10 ml of toluene and 10 ml of pyridine and then heated to95° C. After dissolution, the solution is cooled down to roomtemperature, admixed with 5 ml of water and 50 ml of THF and titratedwith 0.1 N of ethanolic KOH standard solution.

The determination was repeated without test subtance (blank sample).

The acid number was then determined on the basis of the followingformula:

    AN=c.t.56.1.(V1-V2)/m (in mg of KOH/g)

where

c=concentration of ethanolic KOH standard solution in mol/l,

t=titer of ethanolic KOH standard solution

m=weight in mg of test substance

V1=consumption of standard solution with test substance in m1

V2=consumption of standard solution without test substance in m1.

Reagents used:

ethanolic KOH standard solution, c=0.1 mol/l, titer=0.9913 (Merck, Art.No. 9115)

pyridine p.A. (Riedel de Haen, Art. No. 33638)

water, deionized

THF and toluene

(c) Determination of the OH Number

The OH number is the sum of the apparent OH number and the AN:

OH number=apparent OH number+AN

The degree of branching (DB [mol %]) is the molar ratio of the branchingpoints to the sum total from the dicarboxylic acids and dihydroxycompounds.

The film thickness was determined by means of an electronic micrometerscrew.

Tensile tests were carried out in conformance with ISO 527 to determinethe breaking strength (σ_(R) [MPa]), the breaking extension (ε_(R) [%])and the yield stress (σ_(R) [MPa]).

    ______________________________________                                        Abbreviations:                                                                ______________________________________                                        DMT:           dimethyl terephthalate                                         Gl:            glycerol                                                       HDI:           1,6-hexamethylene diisocyanate                                 TBOT:          tetrabutyl orthotitanate                                       TP:            trimethylolpropane                                             ______________________________________                                    

Preparation of Polyesters A1 to A3

Polyester A1

(a') 4672 kg of 1,4-butanediol, 7000 kg of adipic acid and 50 g of tindioctoate were reacted at from 230 to 240° C. under a nitrogenatmosphere. After most of the water formed in the reaction had beendistilled out, 10 g of tetrabutyl orthotitanate (TBOT) were added to thereaction mixture. Once the acid number had fallen below 2, excess1,4-butanediol was distilled out under reduced pressure until the OHnumber reached 56.

(a") 1.81 kg of the polyester from a', 1.17 kg of dimethyl terephthalate(DMT), 1.7 kg of 1,4-butanediol and 4.7 g of TBOT were placed in athree-neck flask and heated under a nitrogen atmosphere with slowstirring to 180° C. During this, the methanol formed in thetransesterification was distilled out. The mixture was heated over thecourse of 2 h, while increasing the stirrer speed, to 230° C. and, aftera further hour, 2 g of 50% by weight aqueous phosphorous acid wereadded. The pressure was reduced to 5 mbar over the course of 1 h, andthe mixture was maintained at 240° C. under a pressure of less than 2mbar for various times in order to monitor the degree ofpolycondensation, during which the excess 1,4-butanediol distilled out.

The polyesters A1 prepared in this way had an acid number of up to 1 mgKOH and an OH number (corr.) of at least 3 mg KOH. The melt flow indicesof polyesters A1 are reported in Table 1.

Polyester A2

776.8 g (4.0 mol) of DMT, 1622.2 g of 1,4-butanediol (18 mol) and 0.327g of TBOT were reacted at from 230 to 240° C. under a nitrogenatmosphere. After most of the methanol formed in the reaction had beendistilled out, 876.8 g (6.0 mol) of adipic acid were added, a vacuum wasapplied and the polycondensation carried on until a melt flow index of60 cm³ /10 min (2.16 kg, 190° C.), an OH number of 5.0 mg of KOH and anacid number of 0.6 mg of KOH was obtained.

Polyester A3

Like polyester A1, except that the condensation was carried on to a meltflow index of 54 cm³ /10 min. Polyester A3 had an acid number of 4.5 mgof KOH and an OH number of 0.9 mg of KOH.

Mixture B1

B1 was prepared by oligomerization of HDI in the presence ofN,N,N-trimethyl-2-hydroxypropylammonium 2-ethylhexanoate. The decreasein the NCO content of the mixture was monitored. At an NCO contentwithin the range from 40 to 43%, the reaction was discontinued bydeactivating the catalyst. The reaction mixture was then distilled.

The composition was determined by gel permeation chromatography.

55% by weight of the mononuclear isocyanurate of hexamethylenediisocyanate,

20% by weight of the binuclear isocyanurate of hexamethylenediisocyanate,

10% by weight of the trinuclear isocyanurate of hexamethylenediisocyanate,

15% by weight of the higher-nuclear isocyanurates of hexamethylenediisocyanate,

Mixture B1 had a total functionality of 3.7 and a number averagemolecular weight Mn of 740 g/mol.

Inventive Examples 1 to 5

The amounts of polyester A1 and mixture B1 stated in Table 1 werecombined at 210° C. under a nitrogen atmosphere and stirred for a fewminutes. The melts were then examined.

Comparative Examples C1 and C2

At the start of step a" in the preparation of polyester A1, the amountof trimethylolpropane (TP) or glycerol (G1) stated in Table 1 was addedto the monomers. Otherwise, polyesters C1 and C2 were prepared asdescribed above.

Comparative Example C3

Preparation took place as described under A1. The polyesters A1 werethen reacted with 1,6-hexamethylene diisocyanate and glycerol instead ofwith a mixture B1. The reaction with 1,6-hexamethylene diisocyanate wascarried out as described under Examples 1 to 5.

Comparative Examples C4 to C6

The procedure was as described under Examples 1 to 5, except that1,6-hexamethylene diisocyanate was employed in place of mixture B1.

                  TABLE 1                                                         ______________________________________                                        Ex.  A1              B1      HDI   TP    Gl                                   No.  [mol-%]/MVI [cm.sup.3 /10 min]                                                                [mol %] [mol %]                                                                             [mol %]                                                                             [mol %]                              ______________________________________                                        1    99.90/5         0.10    --    --    --                                   2    99.92/14        0.08    --    --    --                                   3    99.72/19        0.28    --    --    --                                   4     99.95/139      0.05    --    --    --                                   5    99.72/58        0.28    --    --    --                                   C1   99.72/56        --      --    0.28  --                                   C2    99.91/106      --      --    0.09  --                                   C3    98.22/155      --       0.5  --    0.28                                 C4   >99.5/3         --      <0.5  --    --                                   C5   >99.5/6         --      <0.5  --    --                                   C6   >99.5/26        --      <0.5  --    --                                   C7     100/62        --      --    --    --                                   C8     100/155       --      --    --    --                                   C9      100/>250     --      --    --    --                                   ______________________________________                                    

FIG. 1 shows the viscosity functions (dynamic viscosities) of thepolyesters according to the invention and of the comparative examplesmeasured with oscillatory small-amplitude shear (dynamic stressrheometer DSR with plate-plate geometry, diameter: 25 mm, height: 1 mm)at 140° C. with a shear stress amplitude of 100 Pa.

As is evident from FIG. 1, the effects are most pronounced at shearrates of about 100 rad/s, as typically occur on extrusion of polymermelts.

Inventive Example 6

50 g of polyester A2 were mixed with 0.5 g of mixture B1, whichcorresponds to an addition of 0.0025 mol of isocyanate groups, at 235°C. under a nitrogen atmosphere. The reaction mixture was stirred in akneader throughout the reaction time and the increase in stirrer torquewas measured (see FIG. 2). The melt volume index was 1.2 cm³ /10 min.

Comparative Example C10

Like Inventive Example 6, except that mixture B1 was replaced by 0.2 gof hexamethylene diisocyanate, which likewise corresponds to 0.0025 molof isocyanate groups. The torque increase trajectory is shown in FIG. 3.The melt volume index was 22.3 cm³ /10 min.

The results of these investigations reveal that the polyesters of theinvention have a very much higher viscosity than linear polyesterscontaining the same amount of groups derived from the isocyanates. Thepolyesters of the invention therefore have a higher melt stability.

Example 7

The melt of polyester A3 was combined onto 0.15 mol % of mixture B1 in astatic mixer at 200° C. The resulting polyester of the invention had amelt volume index of 5.3 cm³ /10 min.

Comparative Example C11

The melt of polyester A3 was combined with 0.5 mol % of HDI in a staticmixer at 200° C. The resulting polyester had an MVI of 5.5 cm³ /10 min.

The η* viscosities and the compliances were determined on the melts ofpolyester A3, of polyester 7 according to the invention and of thepolyester which represents the state of the art. The results arereported in Table 2.

The viscosity value η* [Pas] is calculated from the elastic component(G'[Pa]) and the viscous component (G"[Pa]) of the voltage signal whichare measured at an oscillatory shearing of the sample at a certainfrequency (η*=√(G'² +G"²)/ω). The η* reported in the table is that forG"=1000 Pa. The η* values at different temperatures are a measure of thewidth of the temperature window within which a polymer is processible,eg. extrudable, under shear stress. The closer together the η* values atthe different temperatures, the larger the temperature window.

The compliance J_(e) [Pa⁻¹ ] provides information about how flowable apolymer is under processing conditions which involve the action ofshearing forces, for example in an extruder (J_(e) =G'/G"²). A highJ_(e) value indicates a flowable polymer and a high attainablethroughput.

In terms of equipment, the measurements are carried out by applying eachof the vacuum-dried samples at 80° C. to a rotary rheometer (RheometricsPyramic Spectrometer from Rheometric Scientific) equipped with aplate/plate measuring insert, melting it and subjecting the melt to anoscillatory measurement at angular frequencies of from 100 to 0.1 rad/susing a constant shear amplitude of 0.5.

The results of the investigations are reported in Table 2.

    ______________________________________                                               MVI        Temperature                                                 Ex. No.                                                                              [cm.sup.3 /10 min]                                                                       [° C.]                                                                            η* [Pas]                                                                          J.sub.e [Pa.sup.-1 ]                     ______________________________________                                        7      5.3        190        511     9.60E.sup.-05                                              220        266     8.62E.sup.-05                                              250        107     5.97E.sup.-05                            A3     54.1       190        185     1.30E.sup.-05                                              220         85     1.22E.sup.-05                                              250         55     1.11E.sup.-05                            C11    5.5        190        2215    4.60E.sup.-05                                              220        800     3.57E.sup.-05                                              250        240     1.85E.sup.-05                            ______________________________________                                    

Example 8

melt of polyester A3 was combined with 0.15 mol % of mixture B1 in astatic mixer at 200° C. The resulting polyester according to theinvention had an MVI of 14 cm³ /10 min.

Example 9

The melt of polyester A3 was combined with 0.3 mol % of mixture B1 in astatic mixer at 200° C. The resulting polyester according to theinvention had an MVI of 5 cm³ /10 min.

Comparative Example C12

The melt of polyester A3 was combined with 0.35 mol % of HDI in a staticmixer at 200° C. The resulting polyester had an MVI of 15 cm³ /10 min.

Comparative Example C13

The melt of polyester A3 was combined with 0.48 mol % of HDI in a staticmixer at 200° C. The resulting polyester had an MVI of 6 cm³ /10 min.

Comparative Example C14

110.1 kg of the reaction product (a'), from the preparation of polyesterA1, were polycondensed with 87.4 kg of DMT, 117.2 kg of 1,4-butanedioland 0.315 g of TBOT as described under a". The melt was discharged andcombined with 0.35 mol % of HDI in a static mixer at 200° C. Theresulting polyester had an MVI of 12 cm³ /10 min.

Example 10

Like C14, except that the 0.35 mol % of HDI was replaced by 0.3 mol % ofmixture B1. The resulting polyester according to the invention had anMVI of 5 cm³ /10 min.

Inventive polyesters 8 and 9 and also the polyesters A3 and C12 to C14were each extruded at from 130 to 135° C. and blown into a film using ablowup ratio of 1:2. Table 3 shows the results of the application tests.

                  TABLE 3                                                         ______________________________________                                        Ex.  T:A                 min. film                                                                              σ.sub.R                                                                       ε.sub.R                                                                    σ.sub.s                    No.  [mol %] MVI    DB   thickness (μm)                                                                      (MPa) (%)  (MPa)                            ______________________________________                                        A3   40:60   56     0    100.sup.a)                                                                             14    950  5.1                              8    40:60   14     0.17  15      28    710  7.1                              C12  40:60   15     0     25.sup.b)                                                                             24    825  5.9                              9    40:60    5     0.21  15      36    600  7.5                              C13  40:60    6     0     20.sup.b)                                                                             34    640  6.2                              C14  45:55   12     0     15      26    760  7.2                              10   45:55    5     0.20  7       42    580  81                               ______________________________________                                    

a) An intact tubular film could not be produced. The measurement wascarried out on film pieces

b) Films having a thickness of 15 μm could not be produced

T: Proportion of units derived from terephthalic acid

A: Proportion of units derived from adipic acid, each based on the sumtotal of T and A.

Polyesters 8, 9 and A3 and also C12 to C13 differ in construction, buthave the same stoichiometric composition. The data show that thepolyesters of the invention can be processed into thinner films than thecomparative polyesters. Compared with polyesters C12 and C13, thepolyesters of the invention combine similar breaking strengths and yieldstresses with lower breaking extensions.

By means of Comparative Example C14 and Inventive Example 10 it ispossible to determine that the polyesters of the invention have similarmechanical properties to corresponding polyesters containing a higherproportion of aromatic units, or have better mechanical properties forthe same level of aromatic units.

Biodegradability Inventive Example 11

The melt of polyester A3 in a static mixer was admixed with 0.3 mol % ofmixture B1 at 200° C. The resulting polyester had a melt volume index of4.7 cm³ /10 min.

Comparative Example C15

Like Example 11, except that the melt of polymer A3 was mixed with 0.5mol % of HDI instead of with 0.3 mol % of mixture B1. The resultingpolyester had a melt volume index of 4.4 cm³ /10 min.

66 g of polyester 11 and of comparative polyester C15 were separatelyground and mixed with 625 g of compost (36% absolute moisture content)and incubated in an incubator at 58° C. for 105 days. The controlsubstance used was microcrystalline cellulose (Avicells from Merck).

The reactor vessels used had perforated bottom plates, through whichhumidified air was constantly pumped in from below. The compost moisturecontent was adjusted to 55% absolute. The CO₂ concentration resultingfrom the microbial reaction was measured every hour by IR spectroscopyin the exit air and recorded. The gas flow rate determined at eachmeasurement was used to calculate the amount of CO₂ produced per day andthe degree of degradation based on the maximum possible theoretical CO₂formation (ThCo₂). The measured degrees of degradation of three samplesof polyester 11 (PS1, PS2, PS3) and three samples of the controlcellulose (KS1, KS2, KS3) are shown in FIG. 4 together with therespective averages (polyester 11: PSM_(w), cellulose: KSM_(w)).

We claim:
 1. Biodegradable polyesters based onA) 95-99.99 mol % of atleast one polyester containing as monomeric building blocksa₁) an acidcomponent comprisinga₁₁) 20-95 mol % of at least one aliphatic orcycloaliphatic dicarboxylic acid or its ester-forming derivative anda₁₂) 5-80 mol % of at least one aromatic dicarboxylic acid or itsester-forming derivative, the proportions of a₁₁ and a₁₂ being based ineach case on the total amount of a₁₁ and a₁₂, and a₂) at least onedihydroxy compound or at least one amino alcohol or their mixturesand B)0.01-5 mol % of a mixture comprisingb₁) 45.80% by weight of at least onemononuclear isocyanurate (b₁₁) or at least one compound (b₁₂) whichcontains two functional groups capable of reacting with the end groupsof polyester A or mixtures of b₁₁ and b₁₂, b₂) 13-25% by weight of atleast one binuclear isocyanurate (b₂₁) or at least one compound (b₂₂)which contains three functional groups capable of reacting with the endgroups of polyester A or mixtures of b₂₁ and b₂₂, b₃) 5-12% by weight ofat least one trinuclear isocyanurate (b₃₁) or at least one compound(b₃₂) which contains four functional groups capable of reaction with theend groups of polyester A or mixtures of b₃₁ and b₃₂ and b₄) 2-18% byweight of at least one tetra- or higher-nuclear isocyanurate (b₄₁) ormixtures of different isocyanurates (b₄₁).
 2. Biodegradable polyestersas claimed in claim 1 based on97-99.95 mol % of polyester A and 0.05-3mol % of mixture B.
 3. Biodegradable polyesters as claimed in claim 1,wherein B is an isocyanurate mixture comprisingb₁) 45-80% by weight of amononuclear isocyanurate, b₂) 13-25% by weight of a binuclearisocyanurate, b₃) 5-12% by weight of a trinuclear isocyanurate and b₄)2-18% by weight of tetra- or higher-nuclear isocyanurates or theirmixtures.
 4. Molding compositions comprising biodegradable polyesters asclaimed in claim 1 and, if desired, at least one further thermoplasticpolymer or at least one further biodegradable polymer or mixturesthereof.
 5. A process for preparing biodegradable polyesters, whichcomprises, in a first steps polyester A containing as monomeric buildingblocksa₁) an acid component comprisinga₁₁) 20-95 mol % of at least onealiphatic or cycloaliphatic dicarboxylic acid or its ester-formingderivative and a₁₂) 5-80 mol % of at least one aromatic dicarboxylicacid or its ester-forming derivative, the proportions of a₁₁ and a₁₂being based in each case on the total amount of a₁₁ and a₁₂, and a₂) atleast one dihydroxy compound or at least one amino alcohol or theirmixturesbeing prepared and, in a second step, being reacted with amixture B comprising b₁) 45-80% by weight of at least one mononuclearisocyanurate (b₁₁) or at least one compound (b₁₂) which contains twofunctional groups capable of reacting with the end groups of polyester Aor mixtures of b₁₁ and b₁₂, b₂) 13-25% by weight of at least onebinuclear isocyanurate (b₂₁) or at least one compound (b₂₂) whichcontains three functional groups capable of reaction with the end groupsof polyester A or mixtures of b₂₁ and b₂₂, b₃) 5-12% by weight of atleast one trinuclear isocyanurate (b₃₁) or at least one compound (b₃₂)which contains four functional groups capable of reaction with the endgroups of polyester A or mixtures of b₃₁ and b₃₂ and b₄) 2-18% by weightof at least one tetra- or higher-nuclear isocyanurate (b₄₁) or mixturesof different isocyanurates (b₄₁),from 95 to 99.9 mol % of polyester Aand from 0.01 to 5 mol % of mixture B being reacted with one another. 6.A process as claimed in claim 5, wherein from 97 to 99.95 mol % ofpolyester A are reacted with from 0.05 to 3 mol % of mixture B.
 7. Aprocess as claimed in claim 5, wherein mixture B is an isocyanuratemixture comprisingb₁) 45-80% by weight of a mononuclear isocyanurate,b₂) 13-25% by weight of a binuclear isocyanurate, b₃) 5-12% by weight ofa trinuclear isocyanurate and b₄) 2-18% by weight of tetra- orhigher-nuclear isocyanurates or their mixtures.
 8. A process as claimedin claim 5, wherein the reaction of polyester A with mixture B iscarried out in a static mixer.
 9. Moldings, films, fibers or coatingsobtainable from the biodegradable polyesters of claim 1.